Grind surface clean, V out crack, drill ends.
Get 99% or 95% nickle rod.
Weld in short stiches, peening each stich starting with it still red
hot, continue peening until cold. If the workpiece starts getting hot,
take a break until it is cool. Peening stretches the nickle so that it
does not shrink as it cools and cause further cracks.

I wanted to clarify for the OP that I think Ecnerwal meant the common nickel
electrodes 99% or 55%.
I thought that the NiRod would run cooler than say 7018 and therefore needed
less current but found this good
pdf on the subject.
http://content.lincolnelectric.com/pdfs/products/literature/c810.pdf
Take the info with a grain of salt. Any discrepancies can be cleared up here on
the NG. One thing they say
that contradicts what I've learned here is the article says to v-grind all the
way to the bottom of the crack.
That may be counterproductive on thin sections. Your diesel water jacket may
have bunches of extra metal to
play with so your mileage may vary.

Thanks for the link, it proved very informative.
Regarding the engine block, the crack is about 8" long and goes thru two
thick "webs" which are about 3/4" wide and ~1/2" thick. I haven't done any
grinding yet but I suppose the thinner sections are ~ 1/4" thick.
Is Oxy-Acetylene brazing an option here or does an engine block go through
too many thermal stresses for it to hold?
Shawn

Brazing is an execellent fix for cast iron **IF** you can preheat
the whole piece. In your case, you won't be able to get the whole
block up to the necessarey 600F temp. Back to the high nickle
rod. Short bead, peen, take a coffe break, repeat. Should take
you several hours to do your 8" repair.
Shawn wrote:

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